"StumbleUpon, the social discovery engine that was famously acquired by eBay only to take itself private again two years later, is reinventing itself again. The company is rolling out a newly redesigned Web site that features a new logo, new colors and an integrated “Explore Box,” or search engine, that had previously only been available in beta. StumbleUpon has also signed up 250 partners for channels on the site, which will act as verticals users can “follow” in order to get the interesting content they want. The partners include such Web sites as Yelp, Gilt Groupe, Vanity Fair and Funny or Die, as well high-profile names like actor Jim Carrey,athletes Mariano Riveraand Paul Pierce, and late-night TV host Chelsea Handler....